High temperature T1-Ba-Ca-Cu-O and T1-Sr-Cu-O superconductor

ABSTRACT

A new high temperature superconductor with transition temperature above 120 K is disclosed. The superconductor in a preferred embodiment comprises T1RBaCuO wherein R is chosen from Group 2A elements excluding Ba. In another preferred embodiment, the superconductor comprises T1SrCuO. Processes for making high temperature superconductors are also disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to high temperature superconductors.

Recently, there has been much work done on the development of hightemperature superconductors. Recent developments have indicated thatcertain combinations of elements are superconducting. However, thesecompositions typically do not maintain their superconductive attributesat temperatures that exceed approximately 90 K. For example, recently,much work has centered on the use of ternary oxides containing rareearth elements, barium, and copper for superconductivity above thetemperature of liquid nitrogen. But, these systems have limitedtransition temperatures, at or below 93 K, and thus their applicationsare limited.

U.S. patent application Ser. Nos. 082,222, 089,067, and 144,114, filedon Aug. 6, 1987, Aug. 25, 1987, and Jan. 15, 1988, respectively, in thename of the inventors of the present invention disclose superconductorsystems. U.S. patent application Ser. No. 089,067 discloses, in part, asuperconductor based on a R-Ba-Cu-O wherein R is chosen from the groupof rare earth metals excluding praseodymium, cerium, and terbium. U.S.patent application Ser. No. 082,222 discloses, in part, a superconductorbased on a Tb-R-Ba-Cu-O system wherein R is chosen from the group ofrare earth metals excluding praseodymium, cerium, and terbium. U.S.patent application Ser. No. 144,114 discloses, in part, a superconductorbased on a TlBaCuO system.

Although superconductors prepared pursuant to the systems disclosed inthe above-identified patent applications have produced "hightemperature" superconductors and yielded optomistic test results,superconductors prepared pursuant to the present invention have yieldedhigher transition temperatures. Indeed, based on the knowledge of theinventors, superconductors produced pursuant to the present inventionhave yielded the highest transition temperatures to date for anysuperconductor.

A superconductor with a higher transition temperature would be desirablefor many reasons. Such a superconductor would: (1) facilitate thediscovery of the correct theory on oxide superconductivity; (2) providea framework for the search of higher temperature, even room temperaturesuperconductors; (3) allow superconducting components to operate athigher temperatures with lower cost; and (4) provide low cost processingand manufacturability.

Furthermore, many of the superconductor compositions that have beenproposed to date are based on rare earth metals. Due to the short supplyand cost of these rare earth metals, the compositions constructedtherefrom can be quite expensive.

Accordingly, there is a need for improved superconductors with highertransition temperatures.

SUMMARY OF THE INVENTION

The present invention provides improved superconductors with transitiontemperatures above 120 K. Furthermore, the present invention providessuperconductors that contain no rare earth elements.

In an embodiment, the present invention preferably comprises a systemcomprising:

    Tl-R-Ba-Cu-O

wherein: R is a Group 2A element, excluding barium. Preferably, R ischosen from the group of elements consisting of strontium (Sr) andcalcium (Ca).

In another embodiment, the system of the present invention comprises:

    Tl-Sr-Cu-O

In a preferred embodiment, the superconductive system of the presentinvention has the following approximate stoichiometry:

    TICa.sub.y Ba.sub.z Cu.sub.u O.sub.v

wherein:

y is greater than or equal to 0 and less than or equal to 5;

z is greater than or equal to 0 and less than or equal to 5;

y+z is greater than or equal to 0.2 and less than or equal to 5;

u is greater than or equal to 0.5 and less than or equal to 15; and

v is greater than or equal to z+y+u and less than or equal to 2+z+y+u.

In another preferred embodiment, the superconductive system of thepresent invention has the following approximate stoichiometry:

    TlSr.sub.y Ba.sub.z Cu.sub.u O.sub.v

wherein

y is greater than or equal to 0 and less than or equal to 5;

z is greater than or equal to 0 and less than or equal to 5;

y+z is greater than or equal to 0.2 and less than or equal to 5;

u is greater than or equal to 0.5 and less than or equal to 15; and

v is greater than or equal to z+y+u and less than or equal to 2+z+y+u.

In another preferred embodiment, the superconductive system of thepresent invention has the following approximate stoichiometry:

    TISr.sub.y Cu.sub.u O.sub.v

wherein:

y is greater than or equal to 0.2 and less than or equal to 5;

u is greater than or equal to 0.5 and less than or equal to 15; and

v is greater than or equal to y+u and less than or equal to 2+y+u

A method of producing the high temperature superconductor system of thepresent is also provided. In an embodiment, the method allows thesuperconductor to be prepared at temperatures of between approximately850° to about 950° C. in flowing oxygen. The method further allows thesuperconductor to be produced rapidly in about 30 minutes.

Accordingly, an advantage of the present invention is to provide a newsuperconductor with high transition temperatures.

A further advantage of the present invention is to provide a materialsystem that may produce higher temperature superconductors, possiblyeven room temperature superconductors.

A still further advantage of the present invention is that it provides anew high temperature superconductor that is formed at a relatively lowtemperature, and allows for rapid production.

Furthermore, an advantage of the present invention is that it provides amethod for making a new high temperature superconductor.

Still another advantage of the present invention is that it provides amethod for rapidly making a high temperature superconductor.

Yet another advantage of the present invention is that it provides amethod for making a superconductor having a transition temperature above120 K.

Moreover, another advantage of the present invention is that it providesa superconductor system that does not include rare earth metals.

Another advantage of the present invention is that the high temperaturesuperconductor can be used at temperatures near the boiling point ofliquid nitrogen where higher critical currents are to be expected.

Additional advantages and features of the present invention aredescribed in, and will be apparent from, the detailed description of thepresently preferred embodiments and the drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a graph of electrical resistance versus temperaturefor three superconductor samples, the dashed line represents aEu-Ba-Cu-O superconductor and the solid line and line represented by "O"depict samples based on a TI-Ca-Ba-Cu-O superconductor system.

FIG. 2 illustrates a graph of electrical resistance versus temperaturefor two superconductor samples. "" represents a superconductor based onTlSrCuO, while "" represents a superconductor based on TlSrBaCuO.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The present invention provides a new superconductor with transitiontemperature above 120 K. The present invention also provides asuperconductor system that may produce higher temperaturesuperconductors, even room temperature superconductors by furtherelemental substitution and variation of preparation procedures. Inaddition, the present invention provides a method for making these newhigh temperature superconductors.

To date, the inventors of the present invention are not aware of anysuperconductors that have transition temperatures above 100 K. Ofcourse, low transition temperatures limit the applications of thesuperconductors. The inventors of the present invention have discovereda superconductor system that has a transition temperature above 120 K;which is at least 20 K higher than that of any known high temperaturesuperconductor. Because of its higher transition temperature, thepresent invention provides a superconductor that can be operated athigher temperatures with lower cost. Furthermore, the inventors of thepresent invention have found that the high temperature superconductorsystem of the present invention can be rapidly produced at relativelylow temperatures

In an embodiment of the present invention, the system comprises:

    TIRBaCuO

wherein:

R is an element chosen from Group 2A elements excluding barium.

Preferably, R is chosen from the group of elements consisting of calcium(Ca) and strontium (Sr).

In another embodiment of the present invention, the system comprises:

    TISrCuO

In a preferred embodiment, a superconductor based on the newsuperconductive system of the present invention has the followingapproximate stoichiometry:

    TICa.sub.y Ba.sub.z Cu.sub.u O.sub.v

wherein:

y is greater than or equal to 0 and less than or equal to 5;

z is greater than or equal to 0 and less than or equal to 5;

y+z is greater than or equal to 0.2 and less than or equal to 5;

u is greater than or equal to 0.5 and less than or equal to 15; and

v is greater than or equal to z+y+u and less than or equal to 2+z+y+u.

In another preferred embodiment, a superconductor based on the newsuperconductive system of the present invention has the followingapproximate stoichiometry:

    TISr.sub.y Ba.sub.z C.sub.u O.sub.v

wherein:

y is greater than or equal to 0 and less than or equal to 5;

z is greater than or equal to 0 and less than or equal to 5;

Y+z is greater than or equal to 0.2 and less than or equal to 5;

u is greater than or equal to 0.5 and less than or equal to 15; and

v is greater than or equal to z+y+u and less than or equal to 2+z+y+u.

In an embodiment, the superconductor of the present invention has thefollowing approximate stoichiometry:

    TISr.sub.y Cu.sub.u O.sub.v

wherein:

y is greater than or equal to 0.2 and less than or equal to 5;

u is greater than or equal to 0.5 and less than or equal to 15; and

v is greater than or equal to y+u and less than or equal to 2+y+u.

By way of example, and not limitation, examples of the new hightemperature superconductors of the present invention will now be given.

EXAMPLE 1:

A. To create the superconductor of Example 1, the following reagentswere utilized:

1. Tl₂ O₃

2. CaO

3. BaCO₃

4. CuO

B. The following procedure was followed using the above reagents tocreate a superconductor:

1. A mixture of a one molar portion of BaCO₃ and a three molar portionof CuO was ground with an agate mortar and pestle. The ground mixturewas heated in air at approximately 925° C. for more than 24 hours (withseveral intermediate grindings) to obtain a uniform black BaCu₃ O₄powder.

2. The resultant BaCu₃ O₄ powder was mixed with appropriate amounts ofTl₂ O₃ and CaO to obtain a mixture with a nominal composition of Tl₁.86CaBaCu₃ O₇.8+X, which was completely ground, and pressed into a pellet.

3. A tube furnace was heated to a temperature of between approximately850° to about 950° C. with oxygen flowing therein.

4. The pellet was placed in the tube furnace maintaining the temperatureand oxygen flow for approximately 2 to about 5 minutes.

5. The pellet was then taken out of the furnace and quenched in air toroom temperature.

The samples prepared by this procedure had an onset temperature of above120 K, a midpoint of about 110 K, and a zero resistance temperature ofabout 100 K. Qualitative magnetic examinations of the superconductor ofthis example demonstrate a strong Meissner effect, indicating a largevolume fraction of superconducting phase.

EXAMPLE 2:

A. In this example, the following reagents were utilized:

1. Tl₂ O₃

2. CaO

3. BaCO₃

4. CuO

B. To produce a superconductor with these reagents, the followingprocedure was followed:

1. A mixture of a one molar portion of BaCO₃ and a three molar portionof CuO was ground with an agate mortar and pestle, heated in air atapproximately 925° C. for more than 24 hours (with several intermediategrindings) to obtain a uniform black BaCu₃ O₄ powder.

2. The resulting BaCu₃ O₄ powder was mixed with appropriate amounts ofTl₂ O₃ and CaO to obtain a mixture with a nominal composition of Tl₁.86CaBaCu₃ O₇.8+X, which was completely ground, and pressed into a pellet.

3. A tube furnace was heated to a temperature of approximately 850° toabout 950° C. with oxygen flowing therein.

4. The pellet was placed in the tube furnace maintaining the temperatureand oxygen flow for approximately 2 to about 5 minutes.

5. The pellet was taken out of the furnace and quenched in air to roomtemperature.

6. The pellet was then annealed at approximately 450° C. in flowingoxygen for 6 hours.

The samples prepared by this procedure had an onset temperature of above120 K, a midpoint of about 110 K, and a zero resistance temperature ofabout 100 K. FIG. 1 illustrates the resistance-temperature dependencefor a superconductor sample prepared pursuant to this example. Thesample is indicated by a solid line. By way of comparison, asuperconductor based on Eu-Ba-Cu-O system is illustrated by dashedlines. Qualitative magnetic examinations of the samples demonstrated astrong Meissner effect, indicating a large volume fraction ofsuperconducting phase.

EXAMPLE 3:

A. The following reagents were utilized in this example:

1. Tl₂ O₃

2. CaO

3. BaCO₃

4. CuO

B. The following procedure was followed:

1. A mixture of a one molar portion of BaCO₃ and a three molar portionof CuO was ground with an agate mortar and pestle, heated in air at 925°C. for more than 24 hours (with several intermediate grindings) toobtain a uniform black BaCu₃ O₄ powder.

2. The resulting BaCu₃ O₄ powder was mixed with appropriate amounts ofTl₂ O₃ and CaO to obtain a mixture with a nominal composition of Tl₂Ca₁.5 BaCu₃ O₈.5+X, which was completely ground, and pressed into apellet.

3. A tube furnace was heated to a temperature of approximately 850° toabout 950° C. with oxygen flowing therein.

4. The pellet was placed in the tube furnace maintaining the temperatureand oxygen flow for approximately 2 to about 5 minutes.

5. The pellet was then taken out of the furnace and quenched in air toroom temperature.

The samples prepared by this procedure had an onset temperature of above123 K, a midpoint of about 112 K, and a zero resistance temperature ofabout 103 K. Qualitative magnetic examinations of the samples showed astrong Meissner effect, indicating a large volume fraction ofsuperconducting phase.

EXAMPLE 4:

A. The following reagents were utilized in this example:

1. Tl₂ O₃

2. CaO

3. BaCO₃

4. CuO

B. The following procedure was followed in this example:

1. A mixture of a one molar portion of BaCO₃ and a three molar portionof CuO was ground with an agate mortar and pestle, heated in air at 925°C. for more than 24 hours (with several intermediate grindings) toobtain a uniform black BaCu₃ O₄ powder.

2. The resulting BaCu₃ O₄ powder was mixed with appropriate amounts ofTl₂ O₃ and CaO to obtain a mixture with a nominal composition of Tl₂Ca₁.5 BaCu₃ O₈.5+X, which was completely ground, and pressed int apellet.

3. A tube furnace was heated to a temperature of between approximately850° to about 950° C. with oxygen flowing therein.

4. The pellet was placed in the tube furnace maintaining the temperatureand oxygen flow for approximately 2 to about 5 minutes.

5. The pellet was then furnace-cooled to room temperature.

The samples prepared by this procedure had an onset temperature of above123 K, a midpoint of about 112 K, and a zero resistance temperature ofabout 103 K. FIG. 1 illustrates the resistance-temperature dependencefor a superconductor sample prepared pursuant to this example. Thesample is indicated by the line defined by "O" in the figure.Qualitative magnetic examinations of these samples demonstrated a strongMeissner effect, indicating a large volume fraction of superconductingphase.

EXAMPLE 5:

A. In this example, the following reagents were utilized:

1. Tl₂ O₃

2. CaO

3. BaCO₃

4. CuO

B. In this example, the following procedure was followed:

1. A mixture of a one molar portion of BaCO₃ and a three molar portionof CuO was ground with an agate mortar and pestle, heated in air at 925°C. for more than 24 hours (with several intermediate grindings) toobtain a uniform black BaCu₃ O₄ powder.

2. The resulting BaCu₃ O₄ powder was mixed with appropriate amounts ofTl₂ O₃ and CaO to obtain a mixture with a nominal composition of Tl₁.5Ca₀.5 BaCu₃ O₆.8+X, which was completely ground, and pressed into apellet.

3. A tube furnace was heated to a temperature of approximately 850° toabout 950° C. with oxygen flowing therein.

4. The pellet was placed in the tube furnace maintaining the temperatureand oxygen flow for approximately 2 to about 5 minutes.

5. The pellet was then taken out of the furnace and quenched in air toroom temperature.

The samples prepared by this procedure had an onset temperature of about120 K, a midpoint of about 110 K, and a zero resistance temperature ofabove liquid nitrogen temperature.

EXAMPLE 6:

A. The following reagents were utilized in this example:

1. Tl₂ O₃

2. SrCO₃

3. CuO

B. The following procedure was followed in this example:

1. A mixture of a 0.45 molar portion of SrCO₃ and a three molar portionof CuO was ground with an agate mortar and pestle, heated in air at 925°C. for more than 24 hours (with several intermediate grindings) toobtain a uniform black Sr₀.45 Cu₃ O₃.5 powder.

2. The resulting Sr₀.45 Cu₃ O₃.5 powder was mixed with appropriateamount of Tl₂ O₃ to obtain a mixture with a nominal composition of Tl₂Sr₀.45 Cu₃ O₆.5+X, which was completely ground, and pressed into apellet.

3. A tube furnace was heated to a temperature of between approximately1000° to about 1050° C. with oxygen flowing therein.

4. The pellet was placed in the tube furnace maintaining the temperatureand oxygen flow for approximately 2 to about 5 minutes.

5. The pellet was then taken out of the furnace and quenched in air toroom temperature.

Resistance-temperature dependence for a sample prepared pursuant to thisexample is illustrated in FIG. 2 as represented by the line. Althoughresistance did not reach zero above liquid nitrogen temperature, it mayreach zero at lower temperatures. In particular, a decrease in theresistance at about 200 K may originate from the onset of a hightemperature superconducting phase of the sample.

EXAMPLE 7:

A. In this example, the following reagents were utilized:

1. Tl₂ O₃

2. SrCO₃

3. BaCO₃

4. CuO

B. In this example, the following procedure was followed:

1. A mixture of a one molar portion of BaCO₃ and a three molar portionof CuO was ground with an agate mortar and pestle, heated in air at 925°C. for more than 24 hours (with several intermediate grindings) toobtain a uniform black BaCu₃ O₄ powder.

2. The resulting BaCu₃ O₄ powder was mixed with appropriate amounts ofTl₂ O₃ and SrCO₃ to obtain a mixture with a nominal composition of Tl₂SrBaCu₃ O_(8+X), which was completely ground, and pressed into a pellet.

3. A tube furnace was heated to a temperature of between approximately900° to about 950° C. with oxygen flowing therein.

4. The pellet was placed in the tube furnace maintaining the temperatureand oxygen flow for approximately 2 to about 5 minutes.

5. The pellet was then taken out of the furnace and quenched in air toroom temperature.

Resistance-temperature dependance for the sample prepared pursuant tothis example is illustrated in FIG. 2 as the line. Although theresistance did not reach zero above liquid nitrogen temperature, it mayreach zero at lower temperatures. In particular, a decrease inresistance at about 250 K may originate from onset of a high temperaturesuperconducting phase of the sample.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its attendant advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

We claim:
 1. A composition having superconductor properties that has thefollowing approximate stoichiometry:

    TlCa.sub.y Ba.sub.z Cu.sub.u O.sub.v

wherein: y+z is greater than or equal to 0.2 and less than or equal to5; z is greater than 0 and less than 5; y is greater than 0 and lessthan 5; u is greater than or equal to 0.5 and less than or equal to 15;and v is greater than or equal to z+y+u and less than or equal to2+z+y+u.
 2. A composition having superconductor properties that has thefollowing approximate stoichiometry:

    TlSr.sub.y Ba.sub.z Cu.sub.u O.sub.v

wherein: y+z is greater than or equal to 0.2 and less than or equal to5; z is greater than 0 and less than 5; y is greater than 0 and lessthan 5; u is greater than or equal to 0.5 and less than or equal to 15;and v is greater than or equal to z+y+u and less than or equal to2+z+y+u.
 3. A composition having superconductive properties that has thefollowing approximate stoichiometry:

    TlSr.sub.y Cu.sub.u O.sub.v

wherein: y is greater than 0.2 and less than or equal to 5; u is greaterthan or equal to 0.5 and less than or equal to 15; and v is greater thanor equal to y+u and less than or equal to 2+y+u.
 4. A high temperaturesuperconductor comprising a melt produced complex having the nominalcomposition:

    Tl.sub.1.86 CaBaCu.sub.3 O.sub.7.8+X.


5. A high temperature superconductor comprising a melt produced complexhaving the nominal composition:

    Tl.sub.2 Ca.sub.1.5 BaCu.sub.3 O.sub.8.5+X.


6. A high temperature superconductor comprising a melt produced complexhaving the nominal composition:

    Tl.sub.1.5 Ca.sub.0.5 BaCu.sub.3 O.sub.6.8+X.


7. A high temperature superconductor comprising a melt produced complexhaving the nominal composition:

    Tl.sub.2 Sr.sub.0.45 Cu.sub.3 O.sub.6.5+X.


8. A high temperature superconductor comprising a melt produced complexhaving the nominal composition:

    Tl.sub.2 SrBaCu.sub.3 O.sub.8+X.